Rest assured that it's not a BA thread. No one has used one of those on a production engine for at least half a century, particularly Irvine. Anyway if it was they're still available. I have an assortment of BA taps and dies with which to make one.

From what brand and size of engine did said nut originate? Do you have an accurate measuring device with which you can measure the diameter of the Irvine shaft? Which Irvine .40 diesel are we discussing? A pic would be good. There are many different Irvine .40 engines both diesel and glow. Going through the Irvine 40 and 53 engine tests on Sceptre-Flight they all had a 1/4" x 28tpi front shaft thread. Many had a removable front shaft thread.

See: http://sceptreflight.com/Model%20Eng...sts/Index.html